If a little green might help restore Baltimore's ailing harbor, how can a lot be bad? That's the question city, state and federal officials are pondering as they weigh a local marina magnate's plan to fill an unused corner of the Inner Harbor with a large floating marsh.

Inspired by a pair of pint-sized experimental wetlands placed in the harbor two years ago, Dan Naor has proposed building a much larger one, covering 1.6 acres of open water in the Harborview marina off Key Highway. The chief operating officer of Baltimore Marine Centers said it's his way of joining the fight to clean up the city's major tourist attraction, which is blighted by trash, unfit to swim in and beset with algae blooms and fish kills.

"It's almost like, enough is enough," he said. "We really need to do something."

Naor's got space for floating wetlands because marina slip rentals have not recovered since the recession began in 2008. The harbor's condition is part of the problem, he said. Time and again, marina patrons complain to him about the litter and the look and smell of the water.

"We don't see this as an income source," Naor said, but "the true effect, if we can clean the water, we'll get more business."

Assuming he can get the green light for this project, Naor said he could build more, as much as 10 acres of floating beds of rosemallow, sea lavender and salt grass throughout the five marinas he controls in the Inner Harbor.

Floating wetlands could provide some much-needed fish and wildlife habitat in the harbor, which lost its shoreline marshes to development long ago. The floats also may provide at least token help, scientists say, in cleaning up some of the Chesapeake Bay's most polluted water.

But a year after approaching government agencies to seek approval of the Harborview wetland, Naor is still waiting, frustrated by a thicket of questions and concerns raised by federal, state and city officials.

It's not so much the marsh's size that bothers them, but the developer's proposal to build a walkway through the wetland, with three viewing platforms.

Jay Apperson, spokesman for the Maryland Department of the Environment, said state officials generally favor building floating wetlands, but want to avoid or minimize any environmental impacts from building piers and platforms over the water.

The National Marine Fisheries Service likewise said it doesn't want to see piers and platforms cover the water, even if they would be used to teach school groups and others about the ecological role marshes play. Over-water structures should be limited to "water dependent" uses, said federal fisheries officials, who contend classes could learn from the shore.

But Naor called the walkways a central element, as he wants people to experience the wetlands up close. He said he already has approval to put 80 more boat slips there, along with floating docks that would cover nearly as much water as the proposed walkways.

"Let's make a park, bring people in from all over and get them to see what we're trying to do," he said.

City officials, echoing the regulators' general praise, raised other concerns.

"It's a neat concept," said Thomas J. Stosur, the city's planning director. But City Hall wants to be sure the harbor's first large floating wetland is done in a way that doesn't interfere with continued development of the Inner Harbor waterfront.

The city also is worried the wetland may gather litter or otherwise become an eyesore. Naor said he'll make sure it's kept up, but Stosur said city officials want a legally binding maintenance plan — including a pledge to remove it if necessary when the shore is to be redeveloped.

And before any floating marsh goes in, Stosur said, the waterfront promenade that city plans have long called for around the Inner Harbor must be built along this stretch of undeveloped shoreline.

The promenade is a touchy issue for City Hall, as officials earlier this year finally settled a lawsuit aimed at forcing the developer of a Canton townhouse project to install a missing stretch of the brick waterfront walkway. Naor, who also operates a marina at the site of the Canton promenade dispute, was a defendant in that suit.

In this case, Naor said, he only controls the marina at Harborview, not the land, so he can't commit to the promenade. That's not good enough for Stosur, who said he wants to get the city "on firmer ground" about completing this stretch of the promenade.

But Naor and his partners argue the harbor's in such bad shape officials ought to be more willing to experiment. It's impaired by floating trash and plagued by sewage leaks and wet-weather overflows that make most of the Inner Harbor unsafe for swimming. In spring, it's often fouled with algae blooms and fish kills.

Joseph DaVia, a supervisor in the Army Corps of Engineers' Baltimore District office, which must approve anything put in the water, said Naor's proposed wetland is getting "a little more rigorous review" than the earlier floating marshes because it's so much larger. He said federal regulators have been "going back and forth" between city officials and Naor's representatives trying to resolve differences. He couldn't predict when the plan would get acted on.

The project has the backing of a pair of nonprofits working to improve the harbor.

"We can turn this into a big science and school project," said Scott Raymond, vice president for education at the Living Classrooms Foundation, which operates a charter middle school at its east Baltimore campus.

No one thing will to restore the harbor, said David Flores, water quality manager for Blue Water Baltimore, a watershed watchdog group, but the floating wetlands can provide much-needed wildlife habitat, raise public awareness about the harbor and give it a more natural appearance.

"Historically, the harbor was full of wetlands," said Ted Gattino, managing partner of Bluewing Environmental Solutions & Technologies, one of Naor's partners in the project. The local consulting firm provided the base for the small floating wetland put in the water two years ago by the National Aquarium.

Gattino said he's convinced floating wetlands can put a dent in the nutrient pollution afflicting the Inner Harbor and the larger Chesapeake from sewage treatment plants and urban and suburban storm-water runoff. Nitrogen and phosphorus from those sources feed algae blooms every spring that suck the oxygen out of the water and suffocate fish.

The Harborview marsh could extract more than 70,000 pounds of nitrogen and nearly 8,300 pounds of phosphorus from the water annually, Gattino calculated.

Scientists say floating wetlands can provide food and shelter for ducks and other waterfowl, and they'll give fish and crabs some place to hide from predators in a water body devoid of submerged vegetation. They've also proven effective at reducing nutrient pollution in ponds, lakes and reservoirs. Researchers question, though, whether floating wetlands would have much impact on open bodies of water, especially brackish ones like the harbor where tides can wash nutrient pollution in from elsewhere.

Daniel Terlizzi, a University of Maryland water quality specialist working at the Columbus Center in the Inner Harbor, said a student working under him had found some indication the aquarium's pocket wetland reduced nutrient levels in the water around it, but not enough to be sure.

With the benefits of floating wetlands far from settled, William Dennison, vice president for science applications at the UM Center for Environmental Science, suggested a larger experiment is needed, such as Naor's project.

"That's the right place to do it, in the harbor," he said.

The harbor is relatively enclosed, making it easier to assess the wetlands' impact, he pointed out. And the water there is so degraded that anything that could help ought to be tried.

"We don't have a lot to lose here," Dennison said. "This is a system in need of major intervention, let's go for it."

Even if Naor gets the regulators' OK, he still needs to figure out how to pay for the project, which he estimates could cost $6 million. He hopes to tap private, charitable and government sources.